1. Field of the Invention
The present invention relates to an electroluminescent panel (herein after, simply referred to as panel) sealing a light emitting element formed over a substrate between the substrate and a cover member. Further, the invention relates to an electroluminescent module where an IC including a controller is mounted over the panel. Further, in the specification, both of the panel and the electroluminescent module are generally referred to as luminescent device.
2. Description of the Related Art
A light emitting element spontaneously emits light and therefore, having high visibility, dispensing with a backlight needed in a liquid crystal display (LCD), optimum for thin formation and not restricted in viewing angle. Therefore, in recent years, a luminescent device using a light emitting element attract attention as a display device substituting for CRT or LCD.
Further, in the specification, a light emitting element generally includes an element luminance of which is controlled by current or voltage and includes an electron source element (electron discharge element) of MIM type used in OLED (Organic Light Emitting Diode) or FED (Field Emission Display).
OLED which is one of light emitting elements includes a layer including a compound providing electroluminescence generated by applying an electric field (electroluminescent material) (herein after, referred to as electroluminescent layer), an anode layer and a cathode layer. As luminescence in the electroluminescent material, there are luminescence in returning from a singlet excited state to a ground state (fluorescence) and luminescence in returning from a triplet excited state to the ground state (phosphorescence).
The electroluminescent layer specifically includes a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer and an electron transporting layer. OLED is basically constructed by a structure of successively laminated anode/light emitting layer/cathode and, other than the structure, may be constructed by a structure of successively laminated anode/hole injecting layer/light emitting layer/cathode, or anode/hole injecting layer/light emitting layer/electron transporting layer/cathode. Further, an inorganic compound may be included in the layers.
Meanwhile, lowering of luminance of OLED in accordance with a deterioration in an electroluminescent material poses a serious problem in putting light emitting devices into practical use.
FIG. 17A shows a change over time of luminance of a light emitting element when constant current is supplied between two electrodes of the light emitting element. As shown by FIG. 17A, even when constant current is made to flow there between, an electroluminescent material is deteriorated with elapse of time and luminance of the light emitting element is lowered.
Further, FIG. 17B shows a change over time of luminance of a light emitting element when constant voltage is applied between two electrodes of the light emitting element. As shown by FIG. 17B, even when constant voltage is applied there between, luminance of the light emitting element is lowered with elapse of time. It seems that as shown by FIG. 17A, luminance with respect to the constant current is lowered by deterioration of an electroluminescent material and as shown by FIG. 17C, current flowing in the light emitting element when applied with constant voltage is reduced over time.
In most cases, gray scale displayed at each pixel differs by an image and therefore, in the case of a time gray scale system using a digital video signal, a period of emitting light by a light emitting element differs among pixels. Further, even in the case of using an analog video signal, a period of emitting light by a light emitting element and an amount of current supplied to a light emitting element differ among pixels. Therefore, the deterioration of the light emitting element of each pixel differs with elapse of time and luminance is dispersed.
Lowering of the luminance of the light emitting element by the deterioration can be compensated for by increasing current supplied to the light emitting element or increasing drive voltage. However, it is not realistic to provide a power source for supplying voltage or current in correspondence with each pixel and therefore, actually, a common power source for supply voltage or current for all of pixels or a certain number of pixel is provided. When voltage or current supplied from the common power source is simply increased to compensate for lowering of luminance of a light emitting element in accordance with the deterioration, in all of pixelsupplied with the voltage or current, luminance of light emitting elements is increased on an average and a dispersion in luminance among pixels cannot be resolved.
In order to resolve the dispersion of luminance among pixels caused by deterioration, according to Patent reference 1, mentioned below, it is described to maintain luminance of a screen to be equivalent to that before deterioration by counting an accumulated period of lighting a light emitting element and preserving the period in a memory and correcting a video signal based on data of a previously prepared deterioration characteristic.
(Patent Literature 1)
Japanese Patent Laid-Open No. 2002-175041
However, the dispersion of luminance among pixels is not only caused by the deterioration but also by a dispersion in a characteristic of TFTs among pixels as explained below.
In the case of a light emitting device of an active matrix type, current flowing in a light emitting element of each pixel is controlled by a thin film transistor (TFT) similarly provided to each pixel. FIG. 18 shows a circuit diagram of a pixel of general light emitting device. A pixel shown in FIG. 18 includes two TFTs of a switching TFT 5000 and a driving TFT 5001, a light emitting element 5002 and a storage capacitor 5003.
The gate of the switching TFT 5000 is connected to a scanning line 5004. One of the source and the drain is connected to a signal line 5005 and other thereof is connected to the gate of the driving TFT 5001. One of the source and the drain of the driving TFT 5001 is connected to a power source line 5006 and the other thereof is connected to a pixel electrode (anode or cathode) provided to the light emitting element 5002. One of two electrodes provided to the storage capacitor is connected to the power source line 5006 and other thereof is connected to the gate of the driving TFT 5001.
Further, in the specification, connection signifies electric connection unless specified otherwise.
Switching of switching TFT 5000 is controlled by voltage applied to the scanning line 5004. When the switching TFT 5000 is made ON, a video signal inputted to the signal line 5005 is inputted to the gate of the driving TFT 5001. Further, current of an amount in correspondence with the video signal inputted to the gate of the driving TFT 5001 is supplied to the light emitting element 5002 to thereby control luminance of the light emitting element 5002.
When a characteristics of the driving TFT 5001 for supplying current to the light emitting element 5002 are dispersed among pixels, current applied to the light emitting element 5002 is also dispersed. That is, the dispersion in the characteristic of the driving TFT 5001 causes dispersion of the luminance among pixels.
According to technology described in Patent reference 1, dispersion of luminance caused by dispersion of a characteristic of TFT cannot be restrained.